Epidemiological studies show a relationship between the dietary intake or UV-activation of pre-vitamin D and the incidence of colon cancers. This relationship indicates that vitamin D may have a chemopreventive action and animal studies have confirmed the preventive efficacy of vitamin D and its analogues in colon cancer. However in many instances human cancer cells and tumors become resistant to treatment. Another major concern is the high incidence of side effects unrelated to the anti cancer actions of vitamin D. Even though both the cancer- and non-cancer-related actions of vitamin D are mediated via interaction with its nuclear receptor (VDR) it is not clear if they can be separated. Studies that elucidate the precise mechanism(s) whereby vitamin D exerts its anti-cancer effects are of great significance. A detailed understanding of these pathways may lead to the development of agents or dietary regimens that are effective in patients that are resistant to vitamin therapy and/or to the development of treatments with fewer side effects. Preliminary data demonstrates that the wnt/2- catenin/TCF oncogenic pathway, almost universally activated in colon cancer, is a key intermediary in the preventive action of vitamin D and its analogues in colon cancer. Our data shows that vitamin D represses 2-catenin signaling and that 2-catenin activates VDR. Importantly, we find that certain VDR mutants and vitamin D analogues allow interaction with 2-catenin but not other co-activators. This proposal seeks to explore and exploit the concept that the vitamin D pathway can be selectively activated in intestinal cancer cells expressing high levels of activated 2-catenin. Such a strategy would offer the additional benefit of repressing 2-catenin signaling at the same time as VDR is activated. In aim one we propose a series of computational, structural, and mutational analyses to identify structural features of the VDR, which specify its interaction with 2-catenin. In aim two we will screen virtual and real libraries to identify ligands which allow 2-catenin but not other co-activators to bind the VDR. Our goal is to develop vitamin D analogues that can activate VDR only in situations, such as colon cancer, in which 2-catenin is elevated. In aim three we will use transgenic animals to investigate the role of VDR, VDR mutants and 2- catenin specific vitamin D analogues in protection from 2-catenin induced neoplasia. The APC1638 mouse develops spontaneous intestinal cancers and we recently generated APC1638/VDR-/- bigenic animals. We will use APC1638 ,APC1638/VDR-/-, VDR-mutant and azoxymethane treated animals to investigate the ability of vitamin D analogues to support VDR/2-catenin interactions and whether ablation of VDR plays any role in intestinal tumor development. PUBLIC HEALTH RELEVANCE: Colorectal cancer is the second leading cause of cancer-related death in the United States. The development of virtually all colorectal cancer results from mutation in adenomatous polyposis coli (APC) or 2-catenin genes. A wealth of epidemiological and animal model data supports the hypothesis that vitamin D and its analogues (and sunlight) are potent inhibitors of CRC and our own work shows that certain analogues of vitamin D can inhibit the growth of CRC in a 2-catenin-specific manner. We now propose a series of computational, biochemical and animal model studies to explore and exploit the concept that the vitamin D pathway can be preferentially activated in colon cancers which express high levels of activated 2-catenin.